1. Field of the Invention
The present invention generally relates to improvements in light valve projection systems of the Schlieren dark field type and, more particularly to an improved masking of light valve spill light that eliminates spurious light due to diffracted light rays from the edges of conventional masks.
2. Description of the Prior Art
Light valve projection systems of the Schlieren dark field type have been in commercial use for many years and are capable of providing excellent performance. Typical prior art color projection systems of this type are shown in U.S. Pat. Nos. 3,290,436, 3,352,592 and 3,437,746, all of which were issued to W. E. Good et al. The principles of operation of this type of projection system are briefly described with reference to FIGS. 1, 2 and 3 of the drawings.
With reference first to FIG. 1, there is schematically shown a single-gun television light valve assembly comprising a lamp 10, sealed light valve 12, and Schlieren projection lens 14. The sealed light valve 12 comprises a glass envelope which contains an electron gun 16, input slots 18, focus-deflection system 20, a control layer 32 on a rotatable disk 22, and a fluid reservoir 24.
The electron gun 16 generates, from anode aperture 11, an electron beam which is used to "write" charge patterns on the control layer 32. These patterns create surface deformations in the layer and form light diffraction gratings. The electron beam is focused, deflected, and modulated by electrodes 23, 25, 27, and 21. The control layer surface deformations diffract and modulate the light rays passing through the layer 32 and disk 22.
The focus-deflection system 20 comprises three electrode sets each having four orthogonal electrodes, which form three electrode "boxes", referred to as boxes 23, 25 and 27, and a cylindrical electrode 21. The first of these, box 23, is arranged about the aperture in the input window and serves to center and allow pre-deflection of the electron beam. The next two boxes, boxes 25 and 27, have DC and AC voltages applied to them in a manner to achieve a uniformly focused electron beam image of aperture 11 which is scanned across the raster plane on control layer 32. This, in turn, permits the control layer fluid to be modulated uniformly by charge control to produce a uniformly colored projected image. following the focus-deflection boxes 25 and 27 is a drift ring 21 which serves, with a transparent electrode on disk 22, as an element of the final electron lens in the focus-deflection system 20.
Specific examples of light modulating fluids are disclosed in U.S. Pat. Nos. 3,288,927 to Ralph W. Plump, 3,317,664 and 3,317,665 both to Edward F. Perlowski, Jr., 3,541,992 to Carlyle S. Herrick et al, and 3,761,616 issued to C. E. Timberlake. These fluids may include additives as taught by U.S. Pat. Nos. 3,764,549 and 3,928,394 to David A. Orser. In general, the control layer or light modulating fluid is a very special chemical compound, modified with special additives, having the electro-mechanical and visco-elastic properties needed to produce effective control layer properties in the electron beam addressed light valve.
The basic light collection system includes an arc lamp 10, which may be a Xenon lamp, the arc of which is located at the focus of a reflector system, which may be a simple ellipsoidal reflector, as shown, or a compound reflector, as disclosed for example in U.S. Pat. No. 4,305,099 to Thomas T. True et al. The light from the arc is reflected from the reflector through a pair of spaced lens plates having corresponding pluralities of rectangular lenticules arranged in horizontal rows and vertical columns. The first lens plate is shown in FIG. 1 at 28 and the second lens plate is formed on the light input surface of the glass envelope of the light valve 12. The light from the lamp 10 is projected through a color filter plate 26 and the lenticular lens 28 before entering the light valve 12.
The interior surface of the glass envelope of the light valve 12 carries the input light mask in the form of slots 18 which, for example, may be applied by vapor deposition. The input slots 18 are a series of transparent slots and alternating opaque bars in a pattern generally as indicated in FIG. 1. The filtered light rays from the lamp 10 pass into the light valve 12 through these transparent slots. The lenslets of the lenticular lens 28 and the corresponding lenslets, formed on the light input surface of the glass envelope of the light valve 12, form condensing lens pairs which first focus spots of filtered light onto the slots of the light mask and then re-image the light rays onto the control layer raster plane 32. With this arrangement, efficient utilization is made of light from the arc lamp, and uniform distribution of light is produced, in a rectangular pattern, on the light modulating medium or control layer 32.
The Schlieren projection lens 14 includes Schlieren lens elements 29, output color selection bars 30 and a projection lens system 31. The output selection bars 30 are the complement of the input slots 18. That is, on the output bar plate, the bars are optically aligned with the slots of the input slots 18 so that, in the absence of a diffraction of light passing through the control layer 32, light rays are focused and terminated on the bars of the output bar plate. This creates a "dark field" condition, i.e., no light is transmitted in the absence of a modulating signal superimposed on the raster scanning signals applied to the horizontal and vertical deflection plates of the deflection system 20. It should be noted, however, that the electron beam which scans the raster and provides charge to the control layer is a constant current electron beam, there being no modulation of the intensity of the beam produced by the electon gun 16 (other than during the horizontal and vertical retrace intervals when the beam is off).
The lower half of FIG. 1 shows the cross sections of the light body and light valve components. The spectral diagrams at the bottom indicate how the light is prefiltered before entering the light valve.
FIG. 2 is a simplified light valve diagram showing the color selection action of the three basic gratings. The control layer 32 which is supported by the disk 22 (shown in FIG. 1) is illustrated as having three different diffraction gratings for red, green and blue light components. These diffraction gratings may be written individually or simultaneously and normally are actually superimposed but, for purposes of illustration only, they are shown in FIG. 2 as separated on the control layer 32.
In the light valve projection system shown in FIGS. 1 and 2, green light is passed through the horizontal slots of the input bar plate 18 and is controlled by charge generated diffraction gratings formed by modulating the height of the scanned raster lines on the control layer 32. This is done by controlling the amplitude of a high frequency carrier applied to the vertical deflection plates as modulated by the green video signal as shown in FIG. 3. Magenta (red and blue) light is passed through the vertical slots of the input bar plate 18 and is controlled by charge generated diffraction gratings created at right angles to the raster lines by velocity modulating the electron spot as it is scanned in the horizontal direction. In the example shown in FIG. 3, this is done by applying a 16 MHz (12 MHz for blue) signal to the horizontal deflection plates and modulating it with the red video signal as shown in FIG. 3. The grooves created in the control layer 32 have the proper spacing to diffract the red portion of the spectrum through the vertical output slots in plate 30 while the blue portion is blocked. (When the 12 MHz carrier is used, the blue light is passed by the vertical slots in plate 30 and the red light is blocked.)
Thus, three simultaneous and superimposed primary color pictures can be written with the same electron beam and projected to the screen 33 as a completely registered full color picture. Colors are created by writing miniture diffraction gratings within each picture element on the fluid surface by manipulating the single scanning electron beam. These gratings diffract the transmitted light rays away from their terminations at the output bars where they are spatially filtered to let the desired color reach the screen. The amount of light diffracted is dependent on the depth of the gratings formed in the control layer. This technique permits a full color television picture to be written on a single control layer with no need for further registration.
FIG. 3 shows in block diagram form the basic light valve projector circuitry. A composite video signal is supplied to the input of a decoder 34 which provides at its output red, blue and green video signals. These signals are respectively applied to modulators 36, 38 and 40. A grating generator 42 supplies carrier signals which, in the case illustrated, have frequencies of 16 MHz and 12 MHz, respectively, to modulators 36 and 38 and a signal having a frequency of 48 MHz to modulator 40. The outputs of the red an blue modulators 36 and 38 are combined and superimposed on the horizontal deflection signal from the horizontal deflection signal generator 44. The output of the green modulator 40 is superimposed on the vertical deflection signal from the vertical deflection generator 46.
The basic Schlieren dark field light valve projector as schematically illustrated in FIGS. 1, 2 and 3 has evolved over a period of years to be a highly efficient projector producing excellent quality pictures of good color balance and high resolution. A problem has arisen with newer models of the light valve due to the use of a thicker output window which causes a mask, normally on the outside of the output window, to be too far from the focal plane (control layer) to provide total elimination of the re-imaged filtered light which surrounds the diffraction gradings formed on the control layer 32: This lead to an investigation of various locations and designs which culminated in the present invention.
The use of light masks in projection systems is, of course, known in the art. Such masks have been used in Schlieren dark field light valve projectors, as mentioned above. Other examples in the prior art are shown by U.S. Pat. Nos. 3,702,395 to Rosendahl and 3,806,236 to Downing. Both of these patents show examples of projection systems using high intensity light sources. In the Downing patent, a mask plate is positioned immediately adjacent a film. Rosendahl discloses two embodiments. In one, a mask plate is placed at the focal point, while in the other, a mask plate is placed adjacent a transparency. In both the Downing and Rosendahl patents, the purpose of the mask plates is to block stray light. Similar teachings may be found in U.S. Pat. Nos. 1,630,616 to Hill and 2,019,698 to Fischer et al. In particular, Fischer et al shows mask plates on either side of a film.